1. Field of the Invention
The present invention relates to digital printing apparatus and methods, and more particularly to lithographic printing plate constructions that may be imaged on- or off-press using digitally controlled laser output.
2. Description of the Related Art
In offset lithography, an image is present on a plate or mat as a pattern of ink-accepting (oleophilic) and ink-repellent (oleophobic) surface areas. Ink is retained on the oleophilic regions and rejected where the plate is oleophobic. In a dry printing system, the plate is simply inked and the image transferred onto a recording material; the plate first makes contact with a compliant intermediate surface called a blanket cylinder which, in turn, applies the image to the paper or other recording medium. In typical sheet-fed press systems, the recording medium is pinned to an impression cylinder, which brings it into contact with the blanket cylinder.
In a wet lithographic system, the non-image areas are hydrophilic, and the necessary ink-repellency is provided by an initial application of a dampening (or "fountain") solution to the plate prior to or in conjunction with inking. The ink-repellent fountain solution prevents ink from adhering to the non-image areas, but does not affect the oleophilic character of the image areas.
Both dry and wet lithographic printing plates generally comprise a printing surface disposed on some form of support, which may or may not contribute to the pattern of ink receptivity and rejection. For example, as disclosed in U.S. Pat. No. 5,339,737, laser-imageable lithographic printing constructions may include a first, topmost layer chosen for its affinity for (or repulsion of) ink or an ink-abhesive fluid; an imaging layer, which ablates in response to imaging (e.g., infrared, or "IR") radiation, thereunder; and beneath the imaging layer, a strong, durable substrate characterized by an affinity for (or repulsion of) ink or an ink-abhesive fluid opposite to that of the first layer. Ablation of the imaging layer weakens the topmost layer as well. By disrupting its anchorage to an underlying layer, the topmost layer is rendered easily removable in a post-imaging cleaning step, creating an image spot having an affinity for ink or an ink-abhesive fluid differing from that of the unexposed first layer. In this type of construction, as with many traditional photoexposure-type designs, the substrate is a heavy polymeric film that accepts ink and confers needed strength and durability to the construction. The price of these qualities, however, is material cost and the manufacturing capacity for handling such films.
U.S. Pat. Nos. 5,783,364 and 5,807,658 disclose wet and dry lithographic printing members that include metallic inorganic layers. These layers exhibit both hydrophilicity and substantial durability at very thin application levels, and ablatively absorb imaging radiation, thereby facilitating direct imaging without chemical development. They can also be used to form optical interference structures which, in addition to providing color, likewise absorb imaging radiation and ablate in response to imaging pulses. Wet lithographic printing members based on this concept may include a protective layer that provides protection against handling and environmental damage, extends plate shelf life, and entrains debris generated by ablation. The layer washes away during the printing make-ready process, effectively cleaning the plate and disappearing without the need for a separate removal process. Once again, however, these printing members contemplate heavy polymeric substrates.
Some applications require greater dimensional stability than can be conferred by a plastic film. One such application involves special types of web presses, typically used by publishers of newspapers, that do not provide clamping mechanisms to retain printing plates against the plate cylinders. Instead, the leading and trailing edges of each the plate are crimped and inserted into a slot on the corresponding cylinder, so the plate is held against the surface of the cylinder by the mechanical flexion of the bent edges. Film or plastic materials cannot readily provide the necessary shape retention and physical strength to accommodate use in such presses. For example, while it may be possible to produce relatively permanent bends in a polyester substrate using heat-set equipment, such an approach may prove cumbersome and costly. For these applications, the plastic film substrate is typically laminated to a heavy-duty metal support as described, e.g., in U.S. Pat. No. 5,188,032 (the entire disclosure of which is hereby incorporated by reference).
Metal sheets may also be employed directly as substrates, as is typically done with large-sized plates. The dimensional stability of plastic- or film-based plates tends to decrease with size unless the thickness of the substrate is increased; however, depending on the size of the plate, the amount of thickening necessary to retain acceptable rigidity can render the plate unwieldy, uneconomical or both. By contrast, metal substrates can provide high degrees of structural integrity at relatively modest thicknesses.
Metal supports or substrates are, of course, more expensive than their plastic counterparts, and require specialized, heavy-duty processing equipment. Although substantially intact after even long print runs, they are part of the plate structure, integrally bound to the remaining plate layers, and therefore cannot be reused.